VI EVOLUTION IN BIOLOGY 199 
gated, the substance of this germ has a peculiar 
chemical composition, consisting of at fewest four 
- elementary bodies, viz., carbon, hydrogen, oxygen, 
and nitrogen, united into the ill-defined compound 
known as protein, and associated with much 
water, and very generally, if not always, with 
sulphur and phosphorus in minute proportions. 
_ Moreover, up to the present time, protein is known 
only as a product and constituent of living 
matter. Again, a true germ is either devoid of 
any structure discernible by optical means, or, at 
- most, it is a simple nucleated cell. 
In all cases the process of evolution consists in 
a succession of changes of the form, structure, 
and functions of the germ, by which it passes, 
_ step by step, from an extreme simplicity, or rela- 
tive homogeneity, of visible structure, to a greater 
or less degree of complexity or heterogeneity ; 
and the course of progressive differentiation is 
usually accompanied by growth, which is effected 
_ by intussusception. This intussusception, how- 
ever, is a very different process from that imagined 
either by Buffon or by Bonnet. The substance 
by the addition of which the germ is enlarged is 
‘in no case simply absorbed, ready-made, from the 
 not-living world and packed between the elemen- 
tary constituents of the germ, as Bonnet imagined; 
* In some cases of sexless multiplication the germ is a cell- 
agegregate—if we call germ only that which is already detached 
from the parent organism. 
